Fogo Volcano is a polygenetic volcano located in the central part of S. Miguel Island. Five trachytic eruptions and at least three basaltic events occurred in the area during the last 5000 years (Wallenstein, 1999). The last volcanic episode took place in 1563 and comprised a phreatoplinian intracaldera eruption followed by a basaltic flank eruption four days latter. The hydrothermal manifestations in this volcano include fumarolic fields, thermal and CO2 cold springs and several soil diffuse degassing areas.

On February 2002 a CO2 flux station was installed in the northern flank of Fogo Volcano in order to analyse the CO2 soil flux behaviour and to identify seismovolcanic precursors. To evaluate the influence of the environmental variables in the CO2 flux, multiple regression analysis (Draper and Smith, 1981) was applied to the data set obtained between March 2002 and March 2003. The results showed that during that time period 31,3% of the CO2 flux variation could be explained by the environmental monitored variables. The wind speed appears as the main controlling variable with a negative influence in the CO2 flux values (Viveiros, 2003). Rainfall acts in a less clear way and a larger set of data is needed to better constrain this variable.

Applying the proposed model for the time between March 2003 and March 2004, it was found that in some periods the observed CO2 values don’t match with the ones predicted by the regression model. Such anomalies may be related with the increase of seismic activity at the Azores detected during 2003/2004.